Turbochargers are a type of forced induction system. They deliver air, at greater density than would be possible in the normally aspirated configuration, to the engine intake, allowing more fuel to be combusted, thus boosting the engine's horsepower without significantly increasing engine weight. A smaller turbocharged engine, replacing a normally aspirated engine of a larger physical size, will reduce the mass and can reduce the aerodynamic frontal area of the vehicle.
FIG. 1 shows a typical variable geometry turbocharger (10). Generally, turbochargers (10) use the exhaust flow from the engine exhaust manifold to drive a turbine wheel (12), which is located in a turbine housing (14) to form a turbine stage (16). The energy extracted by turbine wheel (12) is translated to a rotating motion which then drives a compressor wheel (18), which is located in a compressor cover (20), to form a compressor stage (22). The compressor wheel (18) draws air into the turbocharger (10), compresses this air and delivers it to the intake side of the engine.
Variable geometry turbochargers typically use a plurality of rotatable vanes (24) to control the mass flow of exhaust gas which impinges on the turbine wheel (12) and control the power of the turbine stage (16). These vanes (24) also therefore control the pressure ratio generated by the compressor stage (22). In engines, which control the production of NOx by the use of High Pressure Exhaust Gas Recirculation (HP EGR) techniques, the function of the vane pack in a variable geometry turbocharger also provides a means for controlling and generating exhaust back pressure.
A plurality of vanes (24) is provided between a generally annular upper vane ring (UVR) (28), and a generally annular lower vane ring (LVR) (30). The assembly consisting of the plurality of vanes (24) and the two vane rings (28, 30) is typically known as the vane pack (26). Each vane (24) rotates on a pair of opposing axles (32), protruding from opposite sides of the vane (24) with the axles (32) on the same centerline. For each vane (24), one of the axles (32) is located in an aperture (34) in the LVR (30), and the other axle (32) is located in an aperture (36) in the UVR (28). The angular orientation of the UVR (28) is set such that the complementary apertures (34, 36) in the vane rings (28, 30) are concentric with the axles (32) of the vane (24). The vane (24) is free to rotate about the centerline of the two axles (32), which is concentric with the now established centerline of the two apertures (34, 36). Each axle (32) on the UVR side of the vane (24) protrudes through the UVR (28) and is affixed to a respective vane arm (38), which controls the rotational position of the vane (24) with respect to the vane rings (28, 30). Typically there is a separate unison ring which controls all of the vane arms (38) in unison. This unison ring is controlled by an actuator, which is typically commanded by the engine electronic control unit (ECU).
The clearance between the rotatable vanes (24), more specifically between the cheeks (40) of the vanes (24), and the inner surfaces (29, 31) of the upper and lower vane rings (28, 30), is a major contributor to a loss of efficiency in both the control of exhaust gas allowed to impinge on the turbine wheel (12) and in the generation of backpressure upstream of the turbine wheel (12). It is desirable to minimize the clearances between the vane cheeks (40) and the complementary inner surfaces (29, 31) of the vane rings (28, 30) and thus increase the efficiency of the vane pack (26). Unfortunately, the increase in efficiency due the side clearances is inversely proportional to the propensity of the vane pack (26) to wear, stick, or completely jam due to thermal deformation in the turbine housing (14) being transferred to the vane pack (26). So the vane pack (26) needs to be accurately placed and constrained within the turbine housing (14) in a manner which minimizes the transference of thermally induced distortion. While internal to the vane pack (26), the noted clearances need to be such that they maximize efficiency while minimizing the potential for sticking, jamming and wear.
In some VTGs, as depicted in FIG. 2, the LVR (30) is constrained against the turbine housing (14) by a plurality of bolts (42). The UVR (28) and the lower vane ring LVR (30) are held together by studs or bolts (44), which serve to apply a clamp load on the vane rings (28, 30), and on a plurality of spacers (46) placed between the vane rings (28, 30), such that the length of the spacer (46) determines the distance between the UVR (28) and the LVR (30), and thus the clearance between the cheeks (40) of the vanes (24) and the inner surfaces (29, 31) of the vane rings (28, 30). The bolts or studs (44) also serve to provide the angular orientation of the apertures (34, 36) in which the axles (32) of the vanes are constrained. However, such studs are difficult to secure so that they do not unscrew when subjected to vibration, especially in situations where there are high temperature (from 740° C. to 1050° C.). Similarly, in a situation where the temperature can range from below freezing to high combustion-like temperatures (from 740° C. to 1050° C.), it is difficult to maintain clamp load via a nut (48) so that the nut (48) does not come loose due to the differences in coefficients of thermal expansion between the materials of the components in the clamp load set. Thus, what may appear to be a simple clamping device (i.e., a nut and bolt) is actually a complicated engineering issue, which typically requires the use of exotic and expensive materials for the components so that the clamp load is maintained over the aforementioned range of temperatures.
In some VTGs, the LVR is constrained against the turbine housing by a plurality of spacers. The spacers include a shaft on opposite ends of the spacer. One shaft is pressed fit into a receiving aperture in the LVR, and the other shaft is press fit into a receiving aperture in the UVR. The spacers are held in place by the frictional force of the press fit between the shafts and apertures. However, during turbocharger operation, such a vane pack has experienced problems in that the spacers become separated from the vane rings.
During the assembly of the vane pack (26), much effort is spent to ensure that the correct components are used and that the correct clamp loads are applied. Thus, there is a need cost-effective and relatively fast way to apply the desired clamp load to a vane pack.